1. Field of the Invention
The present invention relates to heat exchange devices and more particularly to evaporative cooling devices of the type that can cool a primary or product air stream evaporation of a fluid into a secondary or working air stream. Such devices can also operate to provide heat recovery in combination with ventilation.
2. Description of the Related Art
An evaporative cooler is a device that uses the latent heat of evaporation of a liquid to provide cooling. The principle of evaporative cooling has been known for many centuries. For example, a damp cloth placed over an object will keep the object cool by evaporation of liquid from the cloth. By continuously adding liquid to the cloth, the cooling effect may be maintained indefinitely without input of electrical energy. The lowest temperature that can be reached by evaporation of moisture in this way into an air stream defines the wet-bulb temperature for that air. An indirect evaporative cooler makes use of this principle. A product air stream passing over a primary surface of a heat exchange element may be cooled by a working air stream passing over and absorbing moisture from a secondary wetted surface of the heat exchanger.
According to theory, if a quantity of air is cooled by direct evaporation its absolute humidity increases due to the uptake of moisture. Its relative humidity also increases due to its lowered temperature until at the wet bulb temperature it is fully saturated with water vapour. If the air is cooled without direct evaporation however, its absolute humidity remains the same. As its temperature decreases only the relative humidity increases until full saturation of the air is reached at the so-called dew point. The dew point is thus lower than the wet bulb temperature and is in fact defined as the temperature to which a body of air must be cooled to reach saturation or 100% relative humidity. At this point, water vapour in the air condenses.
Attempts have been made to improve on the principle of indirect evaporative cooling by cooling or drying the working air stream prior to evaporation taking place. A particularly convenient way of cooling the working air stream is to feedback a portion of the cooled product air. Such devices are often referred to as dew point coolers as they may lower the temperature of the product air to below its wet bulb temperature and close to the dew point. By optimising the surfaces with which the air streams exchange heat, highly effective heat transfer can be achieved. This has been found especially significant in the case of the heat transfer from the wetted secondary surface. In order to provide moisture to the working air stream, the wetted secondary surface may be provided with some form of liquid supply e.g. in the form of a hydrophilic layer. The presence of such a layer can however result in increased thermal isolation of the secondary surface from the working air stream, thus reducing heat transfer.
A particularly efficient form of dew point cooler is known from PCT publication WO03/091633, the contents of which are hereby incorporated by reference in their entirety. The device uses a membrane having heat transfer elements on its primary and secondary surfaces. These heat transfer elements are in the form of fins and are believed to improve transmission of heat from the primary surface to the secondary surface. The fins act both to directly conduct heat to the membrane and also to break up the various boundary layers that develop in the flow. They also serve to increase the total area available for heat exchange on the relevant surfaces. Further important features of the wetted second surface are known from that document and also from PCT publication WO05/019739, the contents of which are also incorporated by reference in their entirety. Accordingly, by careful choice of the material used as a water retaining layer, optimal evaporation may be achieved without thermal isolation of the secondary surface from the working air stream.
The driving temperature differential between the primary and secondary flows of an evaporative cooler of this type must be very low in order to achieve cooling down to the dew point. As a consequence, in order for good heat transfer to occur, the heat conduction coefficient across the heat exchanger must be high. In the case of WO03/091633, the point of attachment of the fins to the membrane is believed to be an area of poor heat transmission. According to PCT publication WO 03/091648 A, attempts have been made to improve heat transmission by connecting the fins on opposing sides of a membrane directly through the membrane. According to PCT publication WO 01/57461, the fins are formed as convolutions in the membrane itself.
Metals are generally good conductors of heat and a device described in PCT publication WO04/040219 uses a heat sealable metal laminate for forming both the fins and the membrane. These are then heat sealed together. Nevertheless, the adhesive component of the laminate is believed to adversely affect the heat transfer between the fins on opposite membrane surfaces. Furthermore, during the process of connection, the area of the fins actually pressed into engagement with the membrane is generally less than desired. It should also be noted in this context that heat transfer along the membrane is undesired as it can adversely affect the temperature drop between inlet and outlet. For this reason, metal membranes have in the past generally been avoided in dew point cooling devices.
Many other configurations have also been suggested for evaporative cooling devices, all of which require heat transfer through a membrane. The membrane divides the wet region, where liquid is provided for evaporation, from the dry region. A number of constructions by Maisotsenko et al are shown in U.S. Pat. No. 6,581,402, in which primary and working streams across a plate are separated by channel guides. The secondary stream is diverted to the opposite side of the plate and receives heat by evaporation and by heat transfer from the plate.